1. Field of the Invention
The present invention relates to an automatic piston inserting apparatus to be used for automatically inserting a piston assembly, which is a combination of a piston, a piston ring, and a connecting rod, into a cylinder bore formed in a cylinder block, in assembling a V type or in-line type automobile engine. Particularly, the invention is concerned with an automatic piston inserting apparatus using a vision system for determining an increment of movement of a piston, which apparatus aligns a piston with a cylinder bore and inserts the piston into the cylinder bore efficiently, in a short time, and which has a relatively simple construction and can cope with various sizes of pistons without reconstruction or a procedure changing work.
2. Description of the Prior Art
Heretofore, in an automatic piston inserting equipment, teaching operation is conducted for teaching both the position in which a piston has been taken out and the position of the associated cylinder bore to an NC transfer system. However, if there are variations in the positioning accuracy of a cylinder block in which the cylinder bore is formed or in the dimensional accuracy of the work itself, the piston may be caught in the cylinder bore when inserted or the piston and the cylinder bore may be damaged.
Heretofore, when a piston is to be inserted into a cylinder bore and fitting of the two is to be effected using a machine such as a robot, there has been adopted a method using a jig for the insertion or a method wherein the position of the cylinder bore is detected with use of a vision system (a two-dimensional camera), to eliminate a discrepancy between the position of the cylinder bore and the current position of the piston.
For example, in the case where an insertion jig is used (Japanese Patent Laid Open No. 115129/1984), as shown in FIG. 16(a), an insertion jig 01 has an inlet larger than an insertion bore (cylinder bore) 02. From this inlet the insertion jig 01 is tapered or gently curved, leading to an outlet (insertion port) which is positioned on the side opposite to the inlet and which is of the same shape as an inlet portion of the insertion bore 02.
The insertion jig 01 of such a shape is first placed on an opening portion of the insertion bore 02 formed in an object (cylinder block) 05 which has the insertion bore 02 in such a manner that both axes approach each other as close as possible. At this stage, the axis of the insertion jig 01 and that of the insertion bore 02 are not coincident with each other. In this state, an expanding device 03 having plural fingers adapted to be expanded from inside toward outside is inserted into the insertion bore 02 and the plural fingers are expanded outwards, with the result that the insertion jig 01 moves into alignment with the insertion bore 02 (see FIG. 16(b)). In this state the insertion jig 01 is fixed to prevent its movement.
Next, a to-be-inserted object (piston) 04 is pushed in toward the insertion bore 02 while allowing it to follow the taper shape of the insertion jig 01, whereby it is inserted into the insertion bore 02 through the outlet of the insertion jig 01 (see FIG. 16(c)).
However, in such a conventional method using the insertion jig 01, it is necessary to use the insertion jig 01 which conforms to the shape of the insertion bore, and hence it is necessary to provide insertion jigs 01 in a number corresponding to the number of various type of bores. Besides, it becomes necessary to provide means for switching from one insertion jig to another, with consequent complication of operation and increase of cycle time. Further, the expanding device 03 is also needed, and an extra work time is required for an alignment work using the expanding device. Additionally, the jig is originally required to be machined highly accurately and have durability and is expensive. Under these circumstances, in a multi-variety mixed production line for various types of products exceeding three types, the application of the method using the insertion jig 01 is difficult. For this reason, for example in many engine assembling lines as multi-variety mixed production lines, the piston inserting step for a cylinder bore is carried out by a manual operation.
As another method using a jig there is known a method in which a position detecting jig is moved and the position of an insertion bore is detected while making search and trace with use of force control (Japanese Patent Laid Open No. 256526/1992). In this method, however, a to-be-inserted object chucking/inserting device is required to shift jig-part chucking portions from one to the other, thus taking time and causing an error as a result of repeating the said shifting motion many times. As method utilizing a similar trace means there is known a method in which the position of an insertion bore is detected while a to-be-inserted object itself is allowed to contact and follow an insertion bore (Japanese Patent Laid Open Nos. 108108/1993 and 168927/1996). However, both methods involve the problem of damage to parts for example because the to-be-inserted object itself is contacted with the insertion bore to detect the position of the bore.
In the case where a vision system is used, an offset between a piston and a cylinder bore is detected by means of a camera, position data of a piston chucking/conveying robot are corrected, and in a correct robot position the piston is pushed into the cylinder bore. Heretofore, the camera is relatively expensive and has been a cause of cost increase. Besides, the accuracy of the measurement by the camera does not become higher than the resolution thereof, so for detecting the bore position with a high accuracy it is necessary to see the bore in a close-up state. In the case of a large bore, three or more cameras are needed, and thus an attempt to increase the accuracy leads to a further increase of cost.